p275-281

8/22/2019 p275-281

1/7

275RESONANCE March 2013

GENERAL ARTICLE

Plant Growth Promoting Rhizobacteria

Potential Microbes for Sustainable Agriculture

Jay Shankar Singh

Keywords

Agriculture, biofertilizers, PGPR,

sustainable agriculture.

Plant Growth Promoting Rhizobacteria (PGPR) are a group

of bacteria that enhances plant growth and yield via various

plant growth promoting substances as well as biofertilizers.

Given the negative environmental impact of artificial fertiliz-

ers and their increasing costs, the use of beneficial soil micro-

organisms such as PGPR for sustainable and safe agriculture

has increased globally during the last couple of decades.

PGPR as biofertilizers are well recognized as efficient soil

microbes for sustainable agriculture and hold great promise

in the improvement of agriculture yields.

Agriculture contributes to a major share of national income and

export earnings in many developing countries, while ensuring

food security and employment. Sustainable agriculture is vitally

important in todays world because it offers the potential to meet

our future agricultural needs, something that conventional agri-

culture will not be able to do. Recently there has been a great

interest in eco-friendly and sustainable agriculture. PGPR areknown to improve plant growth in many ways when compared to

synthetic fertilizers, insecticides and pesticides. They enhance

crop growth and can help in sustainability of safe environment

and crop productivity. The rhizospheric soil1 contains diverse

types of PGPR communities, which exhibit beneficial effects on

crop productivity. Several research investigations are conducted

on the understanding of the diversity, dynamics and importance

of soil PGPR communities and their beneficial and cooperative

roles in agricultural productivity. Some common examples of

PGPR genera exhibiting plant growth promoting activity are:

Pseudomonas, Azospirillum, Azotobacter, Bacillus, Burkholdaria,

Enterobacter, Rhizobium, Erwinia, Mycobacterium, Mesorhizo-

bium, Flavobacterium, etc. This article presents perspectives on

the role of PGPR in agriculture sustainability.

Jay Shankar Singh is an

Assistant Professor in the

Department of Environ-

mental Microbiology, BB

Ambedkar (Cen tral)

University, Luc know,

Uttar Pradesh, India. He is

orking on the environ-

mental problems related to

ecology of methane (CH4)-

consuming bacteria

(methanotrophs),

environmental remediation

and sustainable agricul-

ture development. He also

has a strong devotion to

teaching and is always

available to share with

others the ever-growing

enthusiasm for his

research field.

1 The soil surrounding plant root

area where intense biological

and chemical activities are influ-

enced by root exudates, and mi-

croorganisms f eeding on the

compounds.

8/22/2019 p275-281

2/7

276 RESONANCE March 2013

GENERAL ARTICLE

PGPR as Biofertilizers

Free-living PGPR have shown promise as biofertilizers. Many

studies and reviews have reported plant growth promotion, in-

creased yield, solubilization of P (phosphorus) or K (potassium),

uptake of N (nitrogen) and some other elements through inocula-

tion with PGPR. In addition, studies have shown that inoculation

with PGPR enhances root growth, leading to a root system with

large surface area and increased number of root hairs.

A huge amount of artificial fertilizes are used to replenish soil N

and P, resulting in high costs and increased environmental pollu-

tion. Most of P in insoluble compounds is unavailable to plants.

N2-fixing and P-solubilizing bacteria may be important for plantnutrition by increasing N and P uptake by the crop plants, and

playing a crucial role in biofertilization. N2-fixation and P-solubi-

lization, production of antibiotics, and other plant growth pro-

moting substances are the principal contribution of the PGPR in

the agro-ecosystems. More recent research findings indicate that

the treatment of agricultural soils with PGPR inoculation signifi-

cantly increases agronomic yields as compared to uninoculated

soils.

Mechanisms of Plant Growth Promotion by PGPR

Several mechanisms have been suggested by which PGPR can

promote plant growth; some important ones are as follows (Table 1):

Box 1.

Plant growth promoting rhizobacteria (PGPR): PGPR (rhizo-biofertilizers) are a group of bacteria that

actively colonize plant roots and enhance plant growth and yield.

Biofertilizer: Composition of living micro-organisms which, when applied to surface of plant, seed, or soil,

colonize the plant rhizosphere or inside the plant body (as endophyte) and promotes plant growth through

enhancing the supply or availability of fundamental nutrients to crop plants.

Sustainable agriculture: Sustainable agriculture involves the successful management of agricultural re-

sources to satisfy human needs while maintaining or enhancing environmental quality without exploiting the

natural resources of future generations.

Many studies and

reviewshave reported

plant growthpromotion, increased

yield, solubilization of

P (phosphorus) or K

(potassium), uptake

of N (nitrogen), etc.

through inoculation

with PGPR.

8/22/2019 p275-281

3/7


GENERAL ARTICLE

Phytohormone Production

The enhancement in various agronomic yields due to PGPR has

been reported because of the production of growth stimulating

phytohormones (Table 2) such as indole-3-acetic acid (IAA),

gibberellic acid (GA3), zeatin, ethylene and abscisic acid (ABA).

PGPR Crop parameters

Rhizobium leguminosarum Direct growth promotion of canola and lettuce

Pseudomonas putida Early developments of canola seedlings, growth

stimulation of tomato plant

Azospirillum brasilense andA. irakense Growth of wheat and maize plants

P. flurescens Growth of pearl millet, increase in growth, leaf

nutrient contents and yield ofbanana (Musa)

Azotobacterand Azospirillum spp. Growth and productivity of canola

P. alcaligenes, Bacillus polymyxa, Enhances uptakeof N, P and K bymaize crop

andMycobacterium phlei

Pseudomonas, Azotobacte rand Stumulates growth and yield of chick pea (Cicer

Azospirillum spp. arietinum)

R. leguminismarum and Improves the yield and phosphor us upt ake in wheat

Pseudomonas spp.

P. putida, P. flurescens, A. brasilense Improves seed germination, seedling growth and

andA. lipoferum yield of maize

P. putida, P. fluoresce ns, P. fluorescens, Improves seed germination, growth parameters of

P. putida, A. lipoferum, A. brasilense maize seedling in greenhouse and also grain yield

of field grown maize

Table 1. PGPR and their ef-

fect on growth parameters/

yields of crop/fruit plants.

Phytohormones PGPR

Indole-3-acetic acid (IAA) Acetobacter diazotrophicus

andHerbaspirillum seropedicae

Zeatin and ethylene Azospirillum sp.

Gibberellic acid (GA3) Azospirillum lipoferum

Abscisic acid (ABA) Azospirillum brasilense

Table 2. Examples of differ-

ent phytohormone-producing

PGPR.

8/22/2019 p275-281

4/7


GENERAL ARTICLE

Recent studies confirm that the treatment of seeds or cuttings with

non-pathogenic bacteria, such as Agrobacterium, Bacillus, Strep-

tomyces, Pseudomonas, Alcaligenes, etc. induce root formationin some plants because of natural plant growth promoting sub-

stances produced by the bacteria. Although the mechanisms are

not completely understood, root induction by PGPR is the ac-

cepted result of phytohormones such as auxins, growth inhibiting

ethylene and mineralization. Environment-friendly applications

in agriculture have gained more importance, in particular in

horticulture and nursery production. The use of PGPR for nursery

material multiplication may be important for obtaining organic

nursery material because the use of all formulations of synthetic

plant growth regulators, such as indole-3-butyric acid (IBA), are

prohibited in organic agriculture throughout the world.

Phosphate Solubilization

Rhizobium and phosphorus (P) solubilizing bacteria are impor-

tant to plant nutrition. These microbes also play a significant role

as PGPR in the biofertilization of crops. These bacteria secrete

different types of organic acids (e.g., carboxylic acid) thus lower-

ing the pH in the rhizosphere and consequently release the bound

forms of phosphate like Ca3

(PO4)

2in the calcareous soils. Utili-

zation of these microorganisms as environment-friendlybiofertilizer helps to reduce the use of expensive phosphatic

fertilizers. Phosphorus biofertilizers could help increase the avail-

ability of accumulated phosphate (by solubilization), increase the

efficiency of biological nitrogen fixation and render availability

of Fe, Zn, etc., through production of plant growth promoting

substances.

Siderophore Production

PGPR are reported to secrete some extracellular metabolites

called siderophores. For the first time Kloepper et al. (1980)reported the significance of siderophores produced by certain

genera of PGPR in plant growth promotion. Siderophores are

commonly referred to as microbial Fe-chelating low molecular

The enhancement in

agronomic yields by

PGPR is due toproduction of growth

stimulating

phytohormones and

the consequent

release of the bound

form of phosphate,

andextracellular

metabolites.

8/22/2019 p275-281

5/7


GENERAL ARTICLE

weight compounds. The presence of siderophore-producing PGPR

in rhizosphere increases the rate of Fe3+ supply to plants and

therefore enhance the plant growth and productivity of crop.Further, this compound after chelating Fe3+ makes the soil Fe3+

deficient for other soil microbes and consequently inhibits the

activity of competitive microbes.

PGPR as Biocontrol Agents

PGPR produce substances that also protect them against various

diseases. PGPR may protect plants against pathogens by direct

antagonistic interactions between the biocontrol agent and the

pathogen, as well as by induction of host resistance. In recent

years, the role of siderophore-producing PGPR in biocontrol ofsoil-borne plant pathogens has created great interest. Microbiolo-

gists have developed techniques for introduction of siderophore-

producing PGPR in soil system through seed, soil or r oot system.

PGPR that indirectly enhance plant growth via suppression of

phytopathogens do so by a variety of mechanisms. These include:

The ability to produce siderophores (as discussed above) thatchelate iron, making it unavailable to pathogens.

The capacity to synthesize anti-fungal metabolites such as

antibiotics, fungal cell wall-lysing enzymes, or hydrogen cya-nide, which suppress the growth of fungal pathogens.

The ability to successfully compete with pathogens for nutri-ents or specific niches on the root; and the ability to induce

systemic resistance.

Among the various PGPRs identified, Pseudomonas fluorescens

is one of the most extensively studied rhizobacteria, because of its

antagonistic action against several plantpathogens. Banana bunchy

top virus (BBTV) is one of the deadly virus which severely

affects the yield of banana (Musa spp.) crop in Western Ghats,Tamil Nadu, India. It has been demonstrated that application of

P. fluorescens strain significantly reduced the BBTV incidence

in hill banana under greenhouse and field conditions. Different

PGPR produce

substances that

also protect themagainst various

diseases

8/22/2019 p275-281

6/7


GENERAL ARTICLE

Table3. PGPR as biocontrol

agents against various plant

diseases.

PGPR Disease resistance

Bacillus pumilus, Kluyvera cryocrescens, Cucumber Mosaic Cucumovirus (CMV) of tomato

B. amyloliquef aciens and B. subtilus (Lycopersicon esculentum)

B. amylolique faciens, B. subtilis and Tomato mottle virus

B. pumilus

B. pumilus Bacterial wilt disease in cucumber (Cucumis

sativus), Blue mold disease of tobacco ( icotiana)

Pseudomonas fluorescens Sheath blight disease and leaf folder insect in rice

(Oryza sativa), Reduce the Banana Bunchy Top

Virus (BBTV) incidence, Saline resistance in

groundnut (Arachis hypogea)

B . subtilis and B. pumilus Downy mildew in pearl millet (Pennisetum

glaucum)

B. subtilis CMVin cucumber

B. cereus Foliar diseases of tomato

Bacillus spp. Blight of bell pepper (Capsicum annuum), Blight

of squash

Burkholderia Maize (Zea mays)rot

B. subtilis Soil borne pathogen of cucumber and pepper (Piper)

Bacillus sp. andAzospirillum Rice blast

Fluorescent Pseudomonas spp. Rice sheath rot (Sarocladium oryzae)

PGPR species as biocontrol agents against various plant diseases

are given in Table 3.

PGPR as Biological Fungicides

PGPR and bacterial endophytes play a vital role in the manage-

ment of various fungal diseases (Box 2). But one of the major

hurdles experienced with biocontrol agents is lack of appropriate

delivery system.

PGPR and bacterial

endophytes play a

vital role in themanagement of

various fungal

diseases.

8/22/2019 p275-281

7/7


GENERAL ARTICLE

Suggested Reading

[1] B R Glick, C L Patten, G Holguin and D M Penrose, Biochemical and genetic

mechanisms used by plant growth promoting bacteria, Imperial College Press,

London. 1999.

[2] A R Podile and K Kishore, Plant growth-promoting rhizobacteria. In: Plant

associated bacteria. Springer Netherlands, pp. 195230, 2007.

Conclusion

Worldwide, considerable progress has been achieved in the area

of PGPR biofertilizer technology. It has been also demonstratedand proved that PGPR can be very effective and are potential

microbes for enriching the soil fertility and enhancing the agricul-

ture yield. PGPR are excellent model systems which can provide

the biotechnologist with novel genetic constituents and bioactive

chemicals having diverse uses in agriculture and environmental

sustainability. Current and future progress in our understanding of

PGPR diversity, colonization ability, mechanisms of action, for-

mulation, and application could facilitate their development as

reliable components in the management of sustainable agricul-

tural systems.

Address for Correspondence

Jay Shankar Singh

Department of Environment

Microbiology

School for Environmental

Sciences, BB Ambedkar

(Central) University,

Raibarely R oad, Lucknow

226025

Uttar Pradesh, India.

Email:

jayshank ar_1@ yahoo.co.i n

Box 2

Bacillu s subt ilis , Pseudomonas chlororaphis, endophytic P. fluo rescens inhibit the growth of stem blightpatho gen Corynespora casiicola. The seed treatment and soil application of P. fluorescens reduce root rot

of black gram caused by Macrophomina phaseolina. Seed and foliar application of P. fluorescens reduce

sheath blight of rice. B. subtilis in peat supplemented with chitin or chitin-containing materials show better

control of Aspergillus niger and Fusarium udum in groundnut and pigeon pea, respectively. Strains of

Burkholder ia cepa cia have been shown to have biocontrol of Fusarium spp.

It has been also

demonstrated and

proved that PGPR

can be very

effective and are

potential microbes

for enriching the soil

and enhancing the

agricultureyield.

p275-281

Documents